1. Field of the Invention
Aspects of the invention relate to a method of and an apparatus for controlling the phase of an alternating current (AC) power provided to a fixing unit, and a method of controlling a heating element of the fixing unit.
2. Description of the Related Art
An electrophotographic image forming apparatus, such as a printer, a copier, a facsimile machine, or a multifunctional product, is typically provided with a photosensitive medium, a development unit, a transfer unit, and a fixing unit. The transfer unit is an apparatus for transferring an image formed on the photosensitive medium by the development unit with a developer onto a recording paper. The fixing unit is an apparatus for fixing the image transferred onto the recording paper. In a typical fixing unit, a fixing roller is provided separately from a transfer roller that feeds the recording paper so that the recording paper fed by the transfer roller can be pressed and moved forward. The surface of the fixing roller can be heated to a predetermined temperature using a heating element, such as a halogen lamp. Halogen lamps are inexpensive, and the method of controlling a halogen lamp is simple. Thus, halogen lamps are widely used as the heating elements of fixing units. However, since the resistance of a halogen lamp itself is low, a high current is input to the halogen lamp when it is first turned on, and when a large amount of power is required, an input voltage is distorted and a flicker phenomenon occurs. Accordingly, to solve these problems, a phase of an alternating current (AC) power provided to a fixing unit is controlled.
FIGS. 1A through 1D are graphs of phase control characteristics of an AC power provided to a typical fixing unit. In general, if an AC power is provided to a fixing unit, a phase control apparatus is used to control the phase of the provided AC power and thereby control the heating time of a heating element that heats the fixing unit. The phase control apparatus divides a waveform corresponding to a half cycle of a waveform of the AC power provided to the fixing unit into equal time intervals, and generates a control pulse signal for each of the equal time intervals. Then, the generated control pulse signals are sequentially provided to the heating element of the fixing unit so that the heating time of the heating element can be controlled.
Referring to FIG. 1A, if five waveforms 100, 110, 120, 130, and 140 each corresponding to a half cycle of an AC power waveform are sequentially provided to a fixing unit, the phase control apparatus divides waveform 100 corresponding to a half cycle of the AC power waveform into five equal time intervals (T1). Then, by providing a control pulse signal generated in each of the five equal time intervals (T1) to the fixing unit, an on or off time of the AC power provided to a heating element is controlled. That is, by using the control pulse signal, the conducting time of a current provided to the heating element of the fixing unit can be adjusted.
When the time interval obtained by dividing the waveform corresponding to the half cycle of the AC power waveform into equal time intervals is T1 as shown in FIG. 1A, if the first waveform 100 corresponding to the half cycle of the AC power waveform is provided to the heating element of the fixing unit and supply of a current to the heating element begins, a control pulse signal 102 is provided at a time T1 later so that a current stops flowing through the heating element. Accordingly, after the AC power is supplied to the heating element of the fixing unit, the energy is supplied only for the time T1. Also, if the second waveform 110 corresponding to the half cycle of the AC power waveform is provided to the heating element, a control pulse signal 112 is provided at a time 2*T1 later so that energy can be supplied to the heating element only for a time 2*T1 time after the second waveform 110 is provided. That is, control pulse signals 102, 112, 122, 132, and 142 are provided to the fixing unit at the time points shown in FIG. 1B so that energy corresponding to the areas (A, B, C, D, E) of the waveform shown in FIG. 1C can be provided to the heating element of the fixing unit.
Thus, if control pulse signals generated at time points in which the changes in the time interval are uniform are provided to the fixing unit to adjust the time for supplying energy to the heating element, the accumulated energy provided to the heating element will have a waveform in which changes in the energy are not uniform as shown in FIG. 1D. For example, when the phase of the waveform corresponding to the half cycle of the AC power waveform is 90°, the energy supplied to the heating element shows rapid changes.
That is, in a typical phase control apparatus, the phase of the AC power supplied to the heating element of the fixing unit is controlled so that the change in the time interval can be uniform. Accordingly, the changes in the energy provided to the heating element are non-uniform. As a result, a distortion in the input voltage and a flicker phenomenon in which power provided to neighboring circuits momentarily flickers due to the non-uniform energy changes can occur, and therefore can badly affect the operation and stability of the neighboring circuits.